This invention relates to a system to detect the location of an underground cable particularly of a submarine cable buried in the seabed or in the riverbed.
In the case where a submarine cable is layed, the practice is observed in the recent years that the cable is buried in the seabed or in the riverbed rather than simply layed thereon, because this is effective to avoid the accidental breakage of the cable due to fishing gears and ankers of vessels and the like nearby the area in which the cable is located. It is sometimes necessary to lift such a submarine cable for repair. One of the presently prevailing systems for lift of such a cable is to employ a special hook having a shape which allows the hook to bite into the seabed, when it is towed along the surface of the seabed in the direction crossing the cable, for the purpose to allow the hook to grapple the cable and to lift the same. However, since the cable is buried tightly in the seabed, this prevailing system is accompanied by a fault in which the hook is inclined to accidentally break the cable, unless the tow is suspended immediately after the hook grapples the cable. Therefore, the hook is required to be towed in an extremely slow pace to make sure that the tow can be stopped as immediately as possible. Although some type of radio navigation system such as Loran is employed to determine the place where a cable is to be layed or buried, the accuracy of the location is not necessarily high, and location error is often a range from several hundred meters to several thousand meters. Therefore, it is actually necessary to have the hook towed for a considerable distance to locate the cable. This naturally causes the fault that a considerable amount of time and labor is required for the attempt to locate the cable. It is notable, however, that if the location, the buried depth and the direction of an buried cable to be located are known prior to the commencement of the attempt for grappling of the cable, it makes it possible to select a type of special hook which is involved with the optimum efficiency with respect to the specific kind of soil forming the specific seabed and also with respect to the specific buried depth, resultantly making it possible to grapple a cable by towing the hook for a less distance in the direction crossing the cable.
A number of efforts have been made for development of techniques to detect the location of and the direction of a cable. One of the presently available techniques for detection of the location of a cable is a system which utilizes a specific change in output of a detector which change is expected, when the detector crosses the cable. However, this system is accompanied by two kinds of faults, the first of which is that since the specific change in output appears only in one moment while the detector is crossing the cable, it is necessary to keep watching the detector during the entire period in which the cable detecting efforts are made, not to miss the specific moment, and the second of which is that it is difficult to make an accurate marking of the location of the cable detected by this system. On the other hand, though repetition of the above mentioned system makes it possible to detect the rough direction of the cable, this of course is involved with a fault in which a considerable amount of time and labor is required.